To increase the size of the field army in response to the threat of invasion, Gaius Marius in 107 BC decided to ignore the census qualification in place and to recruit with no inquiry into the property of the potential soldier. From that time the legions would largely consist of poor citizens (the capite censi or "head count"). This change to recruitment had a very significant effect on the make up of the Roman legions. Unlike previous generations where the soldiers had a home and land to go back to between campaigns, Marius' army was now one of professional soldiers. These were men who would serve their General for 20 - 25 years before retiring. If their General was successful the soldier could look forward to a grant of land, again reinforcing the bond between the General and his soldiers.

Since most of the head count citizens were poor and could not afford to purchase their own armaments as in the past, Marius arranged for the state to supply them with arms. He offered the common people permanent employment for pay as a professional soldier, and the opportunity to gain spoils on campaign. With little hope of gaining status in other ways, the masses flocked to join Marius in his new army. Additionally, Marius granted full Roman citizenship to the citizens of the Italian allies if they fought for Rome and completed a period of service in the Roman army.

With this permanent standing army Marius was able to standardize training and equipment throughout the Roman Legions. Drilling and training took place year round. Marius organized the legions as follows: The total number of men in a full strength legion was about 6000, of whom about 5000 were actual soldiers. The internal organization of a legion consisted of 10 cohorts of 6 centuries each. The century consisted of 80 soldiers supported by 20 noncombatants and led by a Centurion assisted by an Optio. The century was divided again into 10 man units that tented and messed together in camp. The century fought as a unit, marched as a unit and camped as a unit. The century carried with it all the arms and accoutrements required to feed and maintain it as a fighting unit. This reduced the size of the baggage train required as support and made the army much more mobile. Between 2 and 6 legions clubbed together constituted an army. The legions soon were in peak physical condition and discipline, unmatched in the ancient world. This replaced the maniple as the basic combat unit of the Legion.

The three older types of heavy infantry were replaced by a single, standard type of Legionary based on the Principes. During these reforms, the Legions were also organized into permanent cohorts for the first time. Prior to this cohorts had been temporary administrative units or tactical task forces of several maniples. The senior centurion of the legion was called the 'primus pilus', a career soldier and advisor to the legate.To keep baggage trains from becoming too large, Marius had each man carry as much of his own equipment as he could, including his own armor, weapons and 15 days' rations or about 50–60 pounds of load total. To make this easier, he issued each legionary a forked stick to carry their loads on their shoulders. The soldiers were nicknamed "Marius' Mules" due to the amount of gear they had to carry. Finally, the legionary standards of the eagle, wolf, minotaur, horse and boar were reduced to a single standard. The Eagle, representing Jupiter Optimus Maximus, replaced them all as the single symbol or loyalty, duty and pride among the soldiers and became a cult symbol representing Rome itself.

Gaius Julius Caesar (July 12 or July 13, 100 BC – March 15, 44 BC) was nephew to Gaius Marius by marriage. Caesar began the Gallic Wars (58 BC–49 BC) in which he conquered all of Gaul (the rest of current France, with most of Switzerland and Belgium and parts of Germany, effectively western mainland Europe from the Atlantic to the Rhine) and annexed them to Rome. Caesar defeated the Helvetii (in Switzerland) in 58 BC, the Belgic confederacy and the Nervii in 57 BC and the Veneti in 56 BC. In 55 and 54 BC he made two expeditions to Britain and, in 52 BC he defeated a union of Gauls led by Vercingetorix at the battle of Alesia. He recorded his own accounts of these campaigns in Commentarii de Bello Gallico ("Commentaries on the Gallic War").

According to Plutarch and Brendan Woods, the campaign resulted in 800 conquered cities, 300 subdued tribes, one million men sold to slavery and another three million dead in battle fields. Caesar laid the foundation for the expansion of the Roman Empire. He was assassinated in 44BC. Caesar's will revealed that, having no legitimate children, Caesar had adopted his great-nephew Octavius as his son and main heir. Octavius, later named Augustus Caesar, became the first Emperor of Rome.